Such braking force boosting devices are basically known and in use. DE 196 44 880 C1, for instance, follows the principle of permanently supervising the operation of a pneumatic brake booster by comparing it with the pedal force (driver's intention) and of providing an auxiliary energy source in case the required performance is not rendered. The brake system thus features a redundant design.
A disadvantage of the known brake boosters is that only limited boosting energy is available. In principle, the characteristic line of the input-force-to-output-force ratio of a brake booster is linear. The gradient of the characteristic line corresponds to the design-implied boosting factor available. With the maximum pressure difference having been established between working chamber and vacuum chamber, there prevails a condition of maximum boosting, i.e., any further increase in the output force is possible only by means of an additionally applied pedal force.
As the assembly space in the vehicle is very limited it is not feasible to just increase the dimensions of a pneumatic braking force boosting device in order to achieve a greater performance. On the other hand, however, the driving performances of motor vehicles are permanently on the upward move, thus requiring more powerful high-performance brake systems. Ultimately, in any new product it is highly desirable to economize and save cost. In view of this aspect, redundant-design brake systems are not feasible.
It is an object of the present invention to provide a small-dimension, light-weight braking force boosting device which matches the increased performance requirements within the overall life-time of a vehicle and which, at the same time, provides new potential for economizing.
This task is solved by the present invention wherein means are provided for sensing or identifying the point of maximum boosting of the braking force boosting device and in that a signal can be generated which reports the point of maximum boosting. In principle, these means may be based on different principles. In accordance with this invention, a first solution provides to determine a pressure difference or a pressure gradient between a chamber of variable pressure and the atmosphere. This solution makes use of the fact that, in principle, the pressure of a working chamber of a pneumatic brake booster gradually approaches the ambient air pressure in case of ventilation (i.e., in case of a braking operation). As soon as no pressure difference is to be noticed or the pressure gradient is zero, a signal reporting the point of maximum boosting is generated which can be used for connecting up or making available additional boosting means. This enables appropriate counter measures such as supplementing the brake booster with a hydraulic pressure increase which preferably is to be generated by an ABS unit with a hydraulic pump. This procedure is beneficial in that the brake booster can deliberately be constructed with less performance so that it will be sufficient for, e.g., 90% of all braking operations within the life-time of a vehicle. On the contrary, the particularly high performance needed for the remaining 10% of all braking operations, especially for so-called panic braking operations using the highest braking force, will be generated by the other boosting energy source. A further advantage of this invention is that a control of the pressure increase generated by the hydraulic pump also can bring about a control of the additionally applied boosting energy. In other words, it is thus made possible to adapt the variation of the characteristic line more flexibly to the needs of comfort or safety than is possible by means of the known systems.
Another solution, a solution responsive to the difference in travel, provides a means, e.g., in the form of a switch, for sensing or identifying a distance between the valve piston and the control housing. In case of a distance corresponding to the point of maximum boosting, this means provides an operating or actuating signal, in particular for connecting up additional boosting means.
Preferably, a switch is used by means of which an operating signal can be emitted so that a circuit is closed or opened. As an alternative thereto, it is conceivable to provide a sensor, e.g., a Hall sensor emitting a corresponding sensor signal.
According to a further development of this invention, the valve piston abuts on a stop, disposed within the control housing, when the point of maximum boosting has been reached so that there is no distance between the valve piston and the control housing. The means detects the non-existing distance and emits a signal indicative of the point of maximum boosting.
It is understood that this invention cannot be used for a changed rating of a pneumatic brake booster, only. It is likewise conceivable to compensate any defects or failures of a conventional brake booster by means of the described device without leaving the grounds of this invention.